A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconductive shield layer (conductor or strand shield), an insulating layer, a second semiconductive shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. The outer semiconductive shield can be either bonded to the insulating layer or strippable. Additional layers within this construction such as moisture impervious materials are often incorporated.
Polymeric semiconductive shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kilovolt (kV). These shields are used to provide layers of intermediate conductivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconductive materials is typically in the range of 10−1 to 108 ohm-cm when measured on a completed power cable construction using the methods described in ICEA S-66-524, section 6.12, or IEC 60502-2 (1997), Annex C.
Typical shield compositions contain a polyolefin, such as ethylene/vinyl acetate copolymer, conductive carbon black, an organic peroxide crosslinking agent, and other conventional additives, processing aids, and antioxidants. These compositions are usually prepared in granular or pellet form.
The shield composition is, typically, introduced into an extruder where it is co-extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable. The cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals, which crosslink the polymer.
In order to provide a semiconductive shield, it is necessary to incorporate conductive particles (conductive filler) into the composition. Industry is constantly attempting to reduce the conductive filler loading and thus reduce formulation cost while maintaining a sufficient level of electrical conductivity and improve processability through reduced viscosity.